1. Field of the Invention
The present invention relates to tube cutting tools, and more specifically to a method and apparatus for cutting a vent hole in one side of a tube from the inside out and a sufficiently spaced distance from the tube end to produce the vent hole behind a tube sheet in a heat exchanger.
2. Description of the Prior Art
In tubular heat transfer equipment, such as boilers and heat exchangers, where one heat-carrying medium is kept separate from another heat carrying medium by elongated tubes mounted in tube sheets, it is not unusual for tubes to fail during operation. The tube failures are manifested by leaks through the tube wall, which allows the two heat-carrying fluid media to mix. Common industry practice is to design and install heat exchangers with excess tube capacity so that operators can plug both ends of failed tubes to effectively take them out of operation. They can then continue equipment operation without the benefit of the plugged tubes until either the frequency of tube plugging or the number of tubes plugged reduces the heat exchanger capacity to the point where continued operation is uneconomical. The boiler or heat exchanger equipment is then either replaced or retubed. In retubing, the original tubes are removed from the tube sheets and replaced with new ones.
After tubes have been plugged, it is not unusual for the leaks to become sealed by corrosion products, mineral deposits, or process fluids when the pressure differentials in operating tubes are no longer present. As a result, some fluid may become trapped within the tube. Such trapped fluid in a plugged tube creates a hazard during subsequent repair or salvage operations, because heat from an acetylene torch, welder, carbon are cutter, or even an abrasive saw can cause a sufficient pressure build-up in the tube to cause a plug to blow out with potentially lethal force. Furthermore, a tube can contain noxious fumes or fluids that can be injurious when heated and released from the tube.
Consequently, safe plugging of a leaking or otherwise defective tube requires creating an opening in the tube wall behind the tube sheet or the tube sheet that is large enough to ensure the free flow of fluid surrounding the tube into and out of the tube's interior to adequately vent any entrapped fluid and to prevent any pressure build-up in the tube. However, perforating, cutting, or otherwise penetrating the tube behind the tube sheet is not an easy task, because there is no access to the tube wall behind the tube sheet without completely disassembling the heat exchanger. A common practice is to try to pierce the tube wall from the inside by inserting a rod with a hooked, sharp-pointed end into the tube a sufficient distance to reach beyond the tube sheet and then jamming the point into the tube wall. However, this technique is not always successful due to the tube wail thickness, the material composition of the tube, and the dimensions of the tube and tube sheet.
Devices for completely cutting the tube behind the tube sheet are well known in the art and have been used for this purpose. For example, U.S. Pat. No. 752,728 to Ulrich discloses an internal tool for cutting through an entire pipe. However, completely severing the tube behind the tube sheet, while it solves the pressure problem, is not entirely satisfactory, either. A loose end of the severed tube can collide with, vibrate against, and damage adjacent tubes in the heat exchanger.
Internally operating devices are also well known in the art for cutting internal grooves in a cylindrical object, see U.S. Pat. No. 3,178,972 to Beebee, and for cutting perforations in pipes, see U.S. Pat. No. 1,643,394 to Smith and U.S. Pat. No. 2,306,670 to Sutliff. However, none of those devices are particularly practical, easy to use, or effective for the purpose of venting heat exchanger tubes behind a tube sheet, and there is a need for a simple, but practical and effective solution to that problem.